This invention relates to the replication of information carriers and more particularly to a process for replicating information carriers such as compact discs carrying audio and/or video information.
Compact discs are widely used for storage of audio and/or video information recorded as depressions or pits at a high density. The usual size of the standard compact discs is 4.75 inches and a single disc may carry billions of pits representing recorded information. The information recorded on the discs can be obtained from the disc by appropriate scanning techniques such as optical recovery by means of laser beams. The compact discs carrying recorded information are usually made by forming a photoresist layer on a surface of a disc of glass which has a high degree of flatness. The photoresist layer is then irradiated with a laser beam or electron beam modulated in intensity by information signals to be recorded. The photoresist layer is then developed to form depressions or pits corresponding to the information signals. A thin film of silver is deposited by chemical plating on the surface of the photoresist layer in a step known as silver mirror treatment to render the photoresist layer surface electrically conductive. Thereafter a layer of nickel is deposited on the silver film by electroplating and upon separation of the nickel film it carries the image corresponding to the recorded information and serves as a master.
At the present time, compact discs are produced by injection molding with individual discs being formed in a mold cavity. To produce compact discs by the conventional injection molding procedure, a nickel master is placed into a mold cavity and is copied by injecting a moldable plastic into the mold cavity. When the injected plastic resin is frozen, usually in a period on the order of 10 seconds, the molded plastic carrying the image of the nickel master is removed from the mold. The next step in production involves metallizing the molded plastic with aluminum using known techniques. The aluminum is then coated with a protective lacquer or varnish and finally, the compact disc is provided with a hole which is aligned in the center of the data image lines.
In producing compact discs by conventional injection molding it is very important that the injection molding of the plastic and the metallization step be conducted in a clean environment to prevent dust particles from contaminating the image surface before the metal is added. The pit tracks on the disc are of very minute dimensions with the distance between pit tracks being only about 1.6 microns. If the tracks become clogged or covered, reproduction of the recorded information is seriously affected. The number of compact discs which can be produced by conventional injection molding in a given time is relatively low since the injected plastic must be permitted to freeze before it can be removed from the mold cavity. Furthermore, with the conventional production technique the substrate of the disc is limited to a molded plastic and an injection molding grade of plastic is required.